feat: add real-time notification system with Socket.IO client

Implements the frontend notification client for TEC-2217:

1. notifications-api.ts — API client for list, unread-count,
   markAsRead, markAllAsRead endpoints
2. notifications-store.ts — Zustand store for notification state
   (recent list, unread count, dropdown open state)
3. use-socket-notifications.ts — Socket.IO hook that connects with
   httpOnly cookie auth, listens for notification:new events,
   auto-reconnects, and syncs unread count on (re)connect
4. notification-bell.tsx — Bell icon with unread badge + dropdown
   showing 10 most recent notifications with time-ago formatting,
   mark-as-read on click, mark-all-as-read, and "Xem tất cả" link
5. notifications-provider.tsx — Provider wired into locale layout
   (inside AuthProvider) to initialize Socket.IO connection
6. Dashboard header — NotificationBell placed before LanguageSwitcher

Added socket.io-client dependency.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/web/app/[locale]/(dashboard)/layout.tsx

@@ -22,6 +22,7 @@ import {
 import { usePathname } from 'next/navigation';
 import { useTranslations } from 'next-intl';
 import { useState } from 'react';
+import { NotificationBell } from '@/components/notifications/notification-bell';
 import { useTheme } from '@/components/providers/theme-provider';
 import { Button } from '@/components/ui/button';
 import { LanguageSwitcher } from '@/components/ui/language-switcher';
@@ -247,6 +248,7 @@ export default function DashboardLayout({ children }: { children: React.ReactNod
                 {user.fullName}
               </span>
             )}
+            <NotificationBell />
             <LanguageSwitcher />
             <Button
               variant="ghost"